1. Field of the Invention
The present invention relates to compositions for rubber mountings having excellent vibration isolating properties at a relatively high hardness.
The use of the compositions of the present invention can improve the hardness (static modulus of elasticity) without degrading the vibration isolating properties, so that the rubber compositions according to the present invention are suitable for the material for producing small size and light weight of rubber mountings.
2. Description of the Prior Art
In general, vibration transmissibility .tau. is shown by the following equation in a single degree of freedom mounting system in which viscous resistance C and static spring constant k' act to a substance having mass m ##EQU1## .omega.=2.pi.f is angle frequency, .omega..sub.o =.sqroot.k'/m is specific frequency and l=C.omega./k' is loss factor.
The equation (1) approximates to the following equation (2) in a low frequency zone, that is when .omega. approximates to .omega..sub.o EQU .tau..congruent..sqroot.1+1/l.sup.2 ( 2)
While, in a high frequency zone, the equation (1) appoximates to the following equation (3). ##EQU2##
k'+C.omega.=k" is a dynamic spring constant and k"/k' is a ratio of dynamic-to-static modulus.
From the equations (2) and (3), in order to make the vibration isolating property of the rubber mounting, that is the vibration transmissibility smaller, it is necessary that the loss factor is made higher in a low frequency zone and the ratio of dynamic-to-static modulus is made lower in a high frequency zone.
In the case of rubber mountings for automobiles, as the low frequency zone, about 15 Hz of vibration frequency is considered to be important in order to prevent the bodyshake in view of the vibration isolating design and as the high frequency zone, more than 75 Hz of vibration frequency is considered to be important in order to prevent noise closed in room when running at a high speed.
Within the temperature range wherein the rubber mountings are generally used, that is at a temperature from room temperature to 80.degree. C., the temperature dependency of the dynamic modulus of elasticity and the loss factor shows the negative gradient. Accordingly, in order to satisfy the required properties within the temperature range wherein the rubber mountings are generally used, it is merely necessary that the required properties are satisfied at room temperature side (20.degree. C.) with respect to the ratio of dynamic-to-static modulus and at a high temperature side (80.degree. C.) with respect to the loss factor.
The significance of the present invention will be explained hereinafter in view of the material hardness (static modulus of elasticity) and the vibration isolating properties of the rubber mounting.
Since the rubber mountings are used as a part of automobiles and the like, such a rubber part has been heretofore desired to be small size and light weight, so that the rubber compositions having excellent vibration isolating properties even at a relatively high hardness have been demanded. However, in the prior rubber compositions, if the hardness is increased, the vibration isolating properties have been necessarily degraded. Accordingly, for the portion where the severe vibration isolating properties are desired, such as engine mount (front portion) of automobiles, only ones having a low hardness of no greater than 43.degree. have been provided.
When the hardness of the rubber mountings is low, such a rubber material must be a large size in order to restrain the static displacement when a weight of a member applying vibration is loaded, within a given range, so that in the prior rubber mounting compositions, it has never been attained to obtain ones having a small size and light weight.